A giant airplane that can in mid-flight launch a rocket carrying cargo and humans into orbit is the future of space travel, billionaire philanthropist and Microsoft co-founder Paul Allen said.

The first test flight of the ambitious venture by Allen's new company Stratolaunch Systems is not scheduled until 2015, but partners in the project vowed it would revolutionise space travel in the post-space shuttle era.

Using engines from six 747 jets, the biggest airliner ever built would tote a rocket made by SpaceX and be able to launch payloads, satellites, and someday humans into low-Earth orbit, Allen said.

While he declined to say how much he was investing, Allen said it would be more than he spent on SpaceShipOne, which in 2004 was first commercial craft to complete a suborbital flight and reportedly cost about $25 million to develop.

"For the first time since John Glenn, America cannot fly its own astronauts into space," Allen told reporters, referring to the US space shuttle's retirement this year the first man to orbit the Earth aboard Mercury 7 in 1962.

"Today we stand at the dawn of a radical change in the space launch industry," Allen said, vowing greater flexibility than ground-based rocket launches and better cost effectiveness for cargo and manned missions to space.

"It will keep America at the forefront of space exploration and give tomorrow's children something to search for in the night sky and dream about."

Designs for the massive jet, a collaboration with aerospace pioneer Burt Rutan who designed SpaceShipOne, are at an advanced stage, and a hangar is under construction deep in the Mojave desert.

"It is relatively close to building, as soon as we can get a building big enough," said Rutan.

Talks are under way about potential take-off points, since the plane would need a 3650 metre runway, available at larger airports and air force bases.

The aircraft would "use six 747 engines, have a gross weight of more than 544,000 kilograms and a wingspan of more than 117 meters," press materials said.

The plane would take off and while in flight, deploy the rocket and send cargo into low-Earth orbit. The first test flight could take place in 2015, and the first launch could happen by 2016.

Former Nasa administrator Mike Griffin, who is on the board of the Alabama-based Stratolaunch, said the project furthers the goal of making space travel a common endeavour.

"We believe this technology has the potential to someday make spaceflight routine by removing many of the constraints associated with ground launched rockets," Griffin said.

Advantages include the flexibility to launch from a larger number of locations, and potential cargo markets include the communications satellite industry, and Nasa and Department of defense unmanned scientific satellites, Griffin said.

Allen's announcement adds a new company to the race to replace the US space shuttle by offering an alternative made by private industry for carrying humans to low-Earth orbit.

The end of the space shuttle after 30 years has left Russia as the sole nation capable of carrying astronauts to the International Space Station aboard its Soyuz spacecraft, at a cost per seat that will rise to US$63 million in the coming years.

"So if you can come up with - which we believe we can - a manned version of this, we can be very, very competitive with those kinds of fees," Allen said.

"You have a certain number of dreams in your life that you want to fulfill and this is a dream I am very excited about seeing come to fruition."